The candidate, Dr. Ying Taur, an infectious diseases instructor at Memorial Sloan-Kettering Cancer Center, seeks to achieve an academic career as a clinical researcher in the field of infectious diseases. This project proposes a five-year plan where he can take steps to become an independent physician-scientist, under the mentorship of Drs. Eric Pamer and Kent Sepkowitz. This project seeks to study the intestinal flora of patients undergoing allogeneic hematopoietic stem cell transplantation (HSCT), and determine the relationship between flora and the risk for two infections, bacteremia with vancomycin-resistant Enterococcus (VRE) and Clostridium difficile-associated diarrhea (CDAD). This will be done by collecting stool samples serially from patients undergoing HSCT. Using high-throughput sequencing methods, each stool sample will be analyzed in order to characterize the microbial composition. A variety of microbial ecology measures will be calculated and used to describe the intestinal flora. Correlations will be made with risk of VRE bacteremia and CDAD. The specific aims of the project are as follows: (1) Characterize the changes in intestinal flora that occur during HSCT, (2) Identify intestinal flora profiles that impart risk for VRE bacteremia, (3) Identify intestinal flora profiles that impart risk for CDAD. This project will be able to identify the key players of the gut flora, whose presence or absence play an essential role in the development of infection. Armed with a thorough understanding of the etiologic relationship between the gut flora and infectious diseases, it will be possible to design intelligent strategies to prevent, diagnose, or treat these infections in a definitive manner. Throughout the five-year period, the candidate will meet regularly with his mentors, interact with other pertinent experts at MSKCC, and supplement his training with structured meetings, conferences, and coursework. At the end of the proposal period, he will be able to achieve independent status as a clinical researcher in infectious diseases.